The well-head of a production oil or gas well commonly is provided with a number of valves. Some of these valves are present for safety purposes, to block off the flow of oil or gas from the well as and when required. Others are present to permit selection of one or more different passages through which oil or gas can leave the well, or through which various additives can be passed down the well. Commonly, from two to six or more such valves are present at the well-head. The group of valves at the well-head is known colloquially as the "well tree" or "Christmas tree".
During the course of production in the well, it is sometimes necessary to stimulate the well by means of a fracturing technique to yield increased productivity. Fracturing involves the injection into the well of a pressurized fluid, such as water ,brine, foam or the like, which fluid breaks or fractures the oil or gas producing strata down the well.
During the fracturing process, pressures must be elevated in order to cause the rock formations down the well to fracture. Pressures in excess of 7,500 psi are not uncommon during fracturing processes. Pressures of the magnitude found in fracturing processes can usually be tolerated by the well tubing or casing, which extends downwardly from ground level into the well. However, in many cases, the valves which are placed at the well head to form the well tree are not capable of tolerating such pressures and there is a severe risk of rupture if they are subjected to such pressures. Of course, valves which can tolerate such pressures are available, but they are relatively costly, and are not widely used on wells. High pressures are not normally encountered during operations of wells except during fracturing processes, so the provision of valves which would tolerate high pressure is generally considered as an unnecessary expense, particularly since it is usually not known when the valves are installed whether or not the well will eventually be subjected to a fracturing process.
It has been suggested that the danger of a rupture of the well tree valves could be reduced during fracturing processes, and other processes involving high pressure, by inserting a high-pressure tube through the bores of the open valves of the well tree, with the high-pressure tube engaging, at its bottom end, the well tubing in a pressure-tight relationship. At its upper end, the tube would extend beyond the uppermost valve of the well tree, and would be topped by a high-pressure valve. Thus, the high pressure pumping equipment used in fracturing could be connected to the high-pressure valve on the tube, and the high-pressure fluid would then pass through the tube directly into the tubing of the well. This would prevent the contact of the high-pressure fluid with the low-pressure valves of the well-head, thus reducing the chances of rupture.
Although the use of a high-pressure tube in this way has been generally successful, and has resulted in lessened danger in the field, it is still not completely satisfactory. For one thing, the tube and associated high-pressure valve must be guided straight down into the well tree through some sort of cradle. Additionally, hydraulic means are usually necessary to force the high-pressure tube down through the open valves of the well tree. The hydraulic means and the cradle together form a very heavy, cumbersome piece of equipment, and frequently take up so much space that the high-pressure valve at the end of the tubing must be very far away from the ground, where it cannot be reached easily in case of an emergency.